Induction machines such as synchronous machines and asynchronous machines having cage or slip-ring rotors are used in manifold technical applications both in motor operation and also in generator operation. For example, asynchronous and synchronous generators are used in facilities for the decentralized generation of electrical power, such as wind power facilities, shaft generators, flywheel accumulators, and block heating power plants.
At nearly constant speed, synchronous generators and asynchronous generators having short-circuit rotors are capable in principle of outputting the electrical power generated at constant voltage and frequency directly to the three-phase current mains. The advantages of such facilities are simple and robust construction, low maintenance outlay, and high reliability.
Variable-speed facilities, such as asynchronous machines having slip-ring rotors, are capable of outputting an adapted electrical power in the event of variable speed of the generator. The stator of the machine is connected to the mains and the amplitude and frequency of its voltage is predefined by the mains. A variable voltage having variable amplitude and frequency is applied to the rotor by a pulse-width-modulation inverter, which is a component of an inverter. By changing this three-phase system voltage applied to the rotor at variable slip frequency, all possible operating states (e.g., under-synchronous or over-synchronous operation and also motor and generator operation) may be set in this case.
Field-oriented regulation, as is described in EP 0 043 973 A1, for example, is typically used for regulating induction machines. The goal of field-oriented regulation is to implement the decoupling of field-producing and torque-producing components of the current, i.e., for the torque and the magnetization current (such as the rotor flux) to be adjustable independently of one another. This behavior results in asynchronous machines having slip-ring rotors if the rotor current is oriented to the rotor flux space pointer. The regulation occurs in a rotating field coordinate system. For this purpose the rotor position angle and the position of the stator voltage pointer have to be known for the coordinate transformation. The active and reactive power of the induction machine can only be regulated indirectly in this method, and a current control loop by a cascade regulation, which is subordinate to the power control loop, is additionally necessary.
In contrast to field-oriented regulation, the document “Sensorless Multiscalar Control of Double Fed Machine for Wind Power Generators” by Z. Krzeminski, presented at the IEEE Conference Power Conversion in Osaka, Japan, 2002, discloses power regulation for a double-fed asynchronous machine which is based on the regulation of a multiscalar system model. The system model is based on the definition of state variables of the asynchronous machine and associated state equations (differential equations) of the state variables. By using non-linear feedback, the state equations are linearized and the overall system is converted into two linear partial systems, a mechanical partial system and an electrical partial system. The regulation of the mechanical partial system is performed via a PI regulator which regulates the setpoint variable deviations of the state variables, which result as the cross product (i.e., vector product) of stator flux and rotor current, while the regulation of the electromagnetic partial system occurs via a PI regulator which regulates the setpoint variable deviations of the state variables, which result as the scalar product of stator flux and rotor current. To regulate the active and reactive power, it is necessary to superimpose a control loop for regulating the reactive and feedback power in the form of a cascade regulation on each of the control loops for regulating the mechanical and electromagnetic partial systems. In this case, the output variable of the active power regulator represents the setpoint variable for the mechanical control loop and the output variable of the reactive power regulator represents the setpoint variable for the electromagnetic control loop.
In the power regulation according to Krzeminski, the regulation is no longer performed in a rotating field coordinate system, but rather may be performed in a coordinate system fixed on the winding, i.e., in a coordinate system fixed either on the stator or on the rotor. By using the cross and scalar products of stator flux and rotor current, regulation in coordinate systems fixed on the winding becomes possible, since the products contain the complete information about the mutual position of stator flux vector and rotor current vector. The nonlinear feedback allows linearization and therefore simplification of the mechanical and electromagnetic control loops.
However, the power regulation of Krzeminski has some disadvantages. The parameters of the regulation are strongly dependent on the operating point used (e.g., the speed), so that the regulators of the power control loop used in the subordinate mechanical and electromagnetic control loops must be redesigned in the event of different operating points. In addition, the slip range of the regulation is restricted. Finally, unsatisfactory dynamics and stability of the regulation result.